<ul><li>The SMTP rejection code "550 5.4.1 Recipient address rejected: Access denied" is commonly received when a Microsoft-hosted mailbox is no longer valid.</li><li>This rejection code is often misunderstood as an access denial error, but it actually indicates a user unknown/nobody home bounce.</li><li>When subscribers using Microsoft 365 move on to other companies and leave their work email addresses behind, this rejection code is commonly encountered by email newsletters.</li><li>To handle bounces and rejections accurately, it is important to understand that the "550 5.4.1" code refers to invalid addresses rather than access denial.</li></ul>

550 5.4.1? It (most often) means User Unknown

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